More than 90% of the excitatory synapses in the central nervous system occur on dendritic spines. Changes in the structure of these tiny protrusions have long been implicated in learning and memory, though a clear delineation of such morphological changes has awaited the magnification and resolution provided by serial electron microscopy. Long term potentiation (LTP) is an enduring change in synaptic efficacy that is widely studied as a cellular memory mechanism. Several recent studies have shown that elevated calcium concentration in the dendritic spine, which is crucial for the induction of LTP, is regulated independently of that in the parent dendrite. A likely candidate for this independent calcium regulation, is the spine apparatus and smooth endoplasmic reticulum of dendritic spines. This FIRCA application extends the studies of the Parent Grant to delineate alterations in the dendritic spine apparatus and other organelles, with LTP. The aims are: 1) To determine in the hippocampal area CA1 of adult rats whether the spine apparatuses, tubes of smooth endoplasmic reticulum (SER), and/or their association with other organelles, including polyribosomes, mitochondria, microtubules, or PSDs are altered by LTP. Five groups of hippocampal slices will be evaluated to ascertain transient and enduring changes that are specific to LTP, including; 1 and 2) experimental slices showing potentiation when they are fixed at 4 minutes and 4 hours posttetanus by our new microwave-enhanced procedure (Jensen and Harris, 1989, appendix), 3 and 4) experimental slices that are tetanized in the presence of APV to block LTP and fixed at 4 minutes and 4 hours posttetanus, and 3) control slices that are fixed alter receiving an equal number of stimuli spaced so as not to induce LTP. First, a series sample analysis (Sec Harris et al., 1992, appendix), will be conducted to determine the relative frequencies of dendritic spines containing spine apparatuses and other organelles. Then a randomly-selected subpopulation of these dendritic spines will be photographed and reconstructed through serial sections at the highest magnification possible. Complete reconstructions will be made at both the host and the foreign laboratories. 2) At postnatal day 15 to determine whether the "pre-spine" apparatuses described in Harris et al., 1992, are induced by LTP to form the mature spine apparatus. To determine also whether the associations of these immature spine apparatuses with other organelles in the spines are altered by LTP in the immature hippocampus. Health Relatedness: The foreign laboratory is the first to have described the spine apparatus in the normal adult human cortex, and found that its structure is similar to that in the rodent brain. In addition the spine apparatus is grossly distorted in the hypertrophied spines of epitumorous human cortex. Understanding changes in the spine apparatus and other organelles in dendritic spines with LTP will be important for elucidating their role in the human brain.